In-Flight Oxidation Behavior and Splat Morphology of NiCoCrAlY Coatings Deposited by Low-Temperature HVAF

While the use of thermal barrier coatings (TBCs) protects turbine blades in oxidizing environments, the potential of low-temperature high-velocity air-fuel (HVAF) spraying to reduce in-flight oxidation during bond coat deposition remains largely unexplored. This study investigates the effect of low-temperature HVAF process parameters on the microstructure of NiCoCrAlY splats and coating formation on steel substrates. The resulting splats deposited by the use of an internal diameter HVAF torch exhibited craters and surface features indicative of partially melted and plastically deformed particles. Particle velocity measurements revealed a strong correlation between stand-off distance and both splat morphology and particle deposition density. Energy-dispersive spectroscopy (EDS) analyses detected no significant in-flight oxidation. In order to assess the possible nanoscale oxide formation, advanced characterization techniques, including focused ion beam (FIB) cross-sectioning and low-voltage scanning electron microscopy (LV-SEM), were employed. The optimized process parameters were chosen to fabricate the bulk coating, and the quality and microstructure of the final coating were studied. Among the tested conditions, a 50 mm stand-off distance yielded the highest deposition efficiency and lowest porosity, likely due to reduced particle flight time and bow shock effects. These results underscore the critical role of spray parameters in controlling splat behavior and oxide formation, offering new insights into optimizing low-temperature HVAF for advanced bond coat applications in aerospace.

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Originally published at Journal of Thermal Spray Technology (18 April 2026)
By Murilo Sergio Lamana, Aravind Kumar Thoutam, Amirhossein Mahdavi, Andre C. Liberati, Fadhel B. Ettouil, Ritvij Chandrakar, Stephanie Bessette, Nicolas Brodusch, Raynald Gauvin, Ali Dolatabadi, and Christian Moreau